Nature of the Invention
This invention is an improvement in internally steam heated can rolls in a papermaking machine. The invention involves means for reducing heat losses in all forms from the heads, or end enclosures, from the can roll to provide a marked increase in thermal efficiency. The invention is particularly adaptable for modification of existing papermaking machines as all parts can be affixed to the roll by attachment to existing head bolts that bolt the heads onto the cyclindrical shell of the roll. Thus, a minimum of downtime and other expense is expended in order to utilize the invention with respect to existing machines.
The paper industry is one of the largest industries with respect to consumption of energy in the world. A sizable portion of this energy is wasted from the heads of the drying sections of the papermaking equipment.
The mechanism of heat losses from the outside surface of the heads of the associated can rolls is a combination of radiation and various forms of convection. Radiative losses due to the surface emission of heat are about 10% of the total losses. Convection losses are best considered as comprising three components. The primary mode is the convection as a result of external air draft provided to aid in driving out the moisture from a web being dried. A secondary contribution to convection comes from the relative air movement generated by the rotation of the dryer roll head itself, which at moderate operating speeds has a similar magnitude of heat loss as from the radiative losses above. The final component is the natural free convection resulting from the temperature differential between the external surface of the head and the ambient air.
Typically, with a 60 inch diameter can roll operating at 1200 feet per minute using 300.degree. F. steam, the radiative loss, natural convection loss, and convection due to rotation are of similar orders of magnitude and generally total less than about 30% of the total head heat loss. Heat loss from the shaft itself is generally less than about 7%. The balance of the heat loss, which is generally greater than two-thirds of the total loss from the heads, is from the forced convection due to air draft.
As is seen from FIG. 7, herein, the practice of this invention at today's energy cost, can commonly result in annual energy savings of up to $14,000 dollars per head, depending upon precise operating conditions. One papermaking machine may have more than twenty such can rolls.